1. Field of the Invention
The present invention relates to a method for fabricating a semiconductor device. More specifically, the present invention relates to a method for fabricating a semiconductor device including an electrode having a T-shaped cross section.
2. Description of the Related Art
GaAs metal-semiconductor field-effect transistors (MESFETS) and AlGaAs/CaAs high electron-mobility transistors (HEMTs) are used as low-noise amplifiers in a microwave frequency range. A gate electrode in such a MESFET and a HEMT is required to have the characteristics as mentioned below in order to improve the low noise amplification characteristics: (1) the gate length coming in contact with a semiconductor substrate is short; (2) the gate electrode has a low resistance; and (3) the gate electrode reduces a parasitic resistance. For this reason, low-resistance gate electrodes having a T-shaped cross section (i.e., gate electrodes composed of a narrow lower-portion and a wide upper-portion having low resistance) have been conventionally used.
The following method is currently employed for fabricating such a T-shaped electrode: a fine pattern which defines the gate length (i.e., a pattern for forming the lower-portion of the gate electrode) is formed by using a polymethacrylate-type electron beam resist having an especially high resolution. A pattern for forming a low-resistance portion (i.e., a pattern for forming the upper-portion of the gate electrode) is formed by using a novolak type photoresist. Since a pattern for gate electrode wiring has a larger area than that of the pattern for the gate electrode portion, it is impossible to form the pattern for gate electrode wiring by using electron beam exposure as in the case of forming the pattern for the gate. The reason is that the electron beam exposure has insufficient throughput. In addition, the polymethacrylate-type electron beam resist does not react with ultraviolet rays having a long wavelength (i.e., larger than 290 nm) and visible rays, and therefore, a desired pattern cannot be formed. Accordingly, the pattern for the gate electrode wiring is usually formed by the exposure to far ultraviolet rays (having a wavelength of 290 nm or less).
However, when employing such a fabrication method, the polymethacrylate electron beam resist used for forming the pattern for the lower-portion of the gate electrode and the novolak type photoresist used for forming the pattern for the upper-portion of the gate electrode mix with each other, so that an insoluble layer is formed. As a result, the opening dimensions of the pattern used for forming the lower-portion of the gate electrode vary during the step of forming the pattern used for forming the upper-portion of the gate electrode. As a result, this method has a problem in that the pattern used for forming the upper-portion of the gate electrode must be formed while taking into account the variation of the opening dimensions of the pattern used for forming the lower-portion of the gate electrode.
Moreover, since the novolak type photoresist is a positive resist, such fabrication methods require an inversion process, in addition to an exposure process and a development process, in order to undercut the gate electrode into a desired cross-sectional shape (i.e., T-shape). Accordingly, this method has the problem of complicated processes.
Moreover, according to such fabricating methods, as described above, the gate electrode portion is exposed to an electron beam, and the gate electrode wiring portion is exposed to far ultraviolet rays. As a result, two exposure devices are required. Accordingly, this method has a problem in productivity and cost.
Thus, there is a demand for a simplified method for fabricating a semiconductor device including a T-shaped electrode, such that the opening dimensions can be precisely controlled and that excellent productivity and cost performance can be obtained.